My overall goal is to determine how signals from rod photoreceptors influence human color vision. A major thrust of visual science has been to explain how signals from the three types of cone photoreceptors form a set of parallel, color-opponent pathways. Rod signals also influence these cone pathways but how the rod signals combine with signals from the three cone types or with the color-opponent pathways remains unclear. The general question that links these three aims is whether rod signals influence different portions of the color pathways selectively or nonselectively. Aim (1): How do rod signals influence hues produced by the perceptual color-opponent pathways? I will determine whether rod signals influence the color pathways nonselectively or selectively. If the influence is selective, the studies will both identify the specific portions of the color-opponent pathways on which rod signals act, and test the hypothesis that the selectivity stems from differential adaptation of rod signals in portions of the color pathways. I will use two different psychophysical methods, (a) scotopic color contrast and (b) rod-induced shifts of unique hues. Aim (2): How do rod signals influence color discrimination mediated by S-cone opponent color pathways? I will determine whether rod signals have a selective or nonselective effect on color discrimination. l will determine whether rod signals can enhance, not just reduce, color discrimination ability. l will also determine whether an existing model of rod influence on Rayleigh (M- and L-cone mediated) color discriminations can be generalized to tritan (S-cone mediated) color discriminations. Aim (3): How do rod signals influence the major classes of spectrally opponent ganglion cells in the primate retina? I will determine whether rod signals have selective or nonselective effects on the ganglion cells thought to be important for color vision. This will be accomplished by using an in vitro preparation of the macaque retina, already established here by Dr. Dennis Dacey, to record intracellular light responses from midget and small-bistratified ganglion cells (the apparent L/M-cone and S-cone opponent retinal pathways, respectively). We will directly test the hypothesis, derived from psychophysics, that rod and S-cone signals interact strongly in small bistratified ganglion cells and that rod signals interact more strongly with M-center than with L-center midget ganglion cells. The psychophysical and physiological approaches will contribute to a more realistic understanding of human color vision that incorporates the influence of rods as well as cones.